In the field of microwaves, the Fabrication of circuits carrying out electronic functions such as mixers, amplifiers or others, may be carried out on substrates of various kinds by using etched lines. The making of a microwave subassembly consists in putting several functions in series or in parallel, these functions being linked together by interconnection systems such as etched lines or wiring tracks. In general, several functions are carried out on one and the same substrate by using printed circuit type technology which has been matched to the materials and to the specific thicknesses for microwave applications. Thus, as represented schematically in FIG. 1, a microwave electronic module can consist of an etched circuit 1 made on a substrate 2 of a material such as “Teflon” glass, namely “Teflon” woven with glass fibres. This substrate, generally not very rigid and exhibiting a relatively small thickness with respect to its other dimensions, is coated on its lower face with a metal earth plane 3. These electronic modules are usually mounted on a metal package or support 4. Various types of assembly are therefore used to secure the substrate 2 to the metal package or support 4. However, the use of electronic circuits at near-millimetre frequencies, in particular at frequencies of several gigahertz, demands good electrical contact between the earth plane 3 of the substrate 2 situated on the lower face of the substrate and the metal earth of the support 4. To address this constraint, various methods can be used to assemble the substrate onto the metal package or support. Thus, it is possible to use a screwing process which has the advantage of being very simple, easily automated and inexpensive. However, this process is not applicable to millimetre frequencies, since the thickness of the substrate 2 is very slender with respect to these other dimensions, this possibly giving rise to problems of deformation of the circuits etched and/or mounted on the substrate possibly inducing cracks in the solder joints. Moreover, the contact between the earth plane of the substrate and the earth plane of the metal support is very localized. The other technique widely used in the field of microwaves is a bonding technique which makes it possible to obtain reliable transfer having good electrical characteristics. Several methods of bonding exist for assembling two materials. Thus, mention may be made of the use of a film which makes it possible to achieve assembly of very high reliability, although its material cost is relatively high and it is complex to implement since it requires the application of pressure throughout the adhesive curing cycle. Another technique consists in employing one- or two-component conducting epoxy adhesives, the conducting particles usually consisting of silver or gold. In this case, the adhesive is deposited using, for example, a syringe. Whereas this technique is less expensive than the previous one, it requires good control of the adhesive so as to deposit a layer which is quasi-uniform in terms of thickness and surface. According to another embodiment, the epoxy adhesive may be deposited by screen printing, thereby making it possible to obtain a uniform layer in terms of thickness and surface. This technique also has the advantage of being fast but uses a considerable quantity of adhesive.
In the field of microwaves, one of the abovementioned bonding techniques is used by spreading the paste over the entire surface of the circuit to be bonded so as to obtain good earth transfer, the adhesives being laden with conducting particles of the silver or gold type. The above technique has the drawback of being especially expensive owing to the appreciable quantity of adhesive required. The applicant, during studies carried out on the bonding of substrate comprising at least one microwave electronic module on a metal package or support, has noticed, contrary to the given teachings, that it was not necessary to deposit the adhesive on the entire surface of the substrate. The adhesive can be deposited at specific sites making it possible to avoid non-conductivity of the signal without giving rise to problems in the operation of the electronic module.